Growth factors initially mediate their effects on target cells by binding to specific surface membrane receptors. The mechanism(s) involved in triggering and sustaining transmembrane signals generated by this interaction is central to the issue of regulation of cell growth and differentiation and in regulation of membrane receptor expression in general. The nature of this ligand-receptor interaction for Interleukain-3 (IL-3), a requisite multipotential regulator of bone marrow cell growth, can now be studied effectively in detail because of the availability of purified IL-3 receptor and the mechanism of ligand-receptor transmembrane signalling. Quantitative isolation of IL-3 receptor for monoclonal antibody production, limited amino acid sequencing and gene cloning will be carried out. Once the gene for the IL-3 receptor at various stages of cell growth and differentiation and determine whether the IL-3 receptor gene may be differentially expressed or even altered in cell types whose growth is independent of IL-3, i.e. as a result of retroviral transformation. By analyzing the IL-3 receptor gene and its product, I plan to better understand its method of regulation, to begin to dissect the functional organization of the molecule, and to examine the biochemical role of the IL- 3 receptor in normal progenitor cell growth and differentiation.